BORON CURING AGENTS FOR EPOXY RESINS D A N I E L T . H A W O R T H ' A N D G I L B E R T F. P O L L N O W Z Research Laboratories, Allis-Chalmers .Wanujactacluring Co., Mzlwaukee 7, Wis.
Various boron-containing compounds have been investigated as curing agents for epoxy resins with the purpose of increasing the thermal stability of the resultant polymers. The compounds investigated include the borazines, the monosubstituted boric acids, the boroxines, and the polyborates. Data presented indicate that some of the cured resins possess physical properties which would b e useful in the plastics industry. In particular, a boroxine-amine complex shows considerable promise as a homogeneous, latent during agent.
v'
boron-containing compounds have been examined for their ability to cure epoxy resins. T h e boron compounds were selected for this work in order to extend the thermal stability of the resultant polymers. Many epoxy compounds are commercially available today. most of them having two terminal epoxy groups per molecule. T h e most widely used resins of this type are the diglycidyl ether homologs of epichlorohydrin and bisphenol A :
substituted bo1 ic acid, the boroxines. and the polyborates. T h e resin used in this work and previously described \ V d S Epon Resin 828 (manufactured bv the Shell Chemical Co.). Satisfactory cures were also obtained when these boron compounds were used Lvith Lnion Carbide's E P 201 (3.4-epoxy-6-methylcyclohexylmethyl-3.4-epoxy-6-methylcyclohexanecarboxylate) or Dow-Corning's Sylkem 90 { 1.3-bis [3-(2,3-epoxypropoxy) propyl] tetramethyl disiloxane
4RIOUS
r
1.
1 EP 201 CH3
\/
0
T h e major portion of the work reported herein was performed on the first compound of the series, for which n has an average value of zero. T ~ v otypes of boron-containing compounds, such as the boron trifluoride complexes of various amines and the borate esters, have been used as curing agents for epoxy resins (4?70> 7 7). .4mong the former, the boron trifluoride-monoethylamine complex has been used in commercial applications. When used in catalytic: amounts (3 to 5 parts per hundred of resin. p.h.r.). it acts as a latent curing agent which yields polymers possessing heat-deflection temperatures in the range of 150' to 175" C. Among the borate esters, the triethanolamine borate has been suggested as a superior catalyst for epoxy resins with a good pot life; however: long cures are required and gelation is slow. Gelation can be hastened by adding an accelerator such as chromium acetylacetonate. Data published by Lee and Neville on the use of boron compounds as epoxy curing agents ( 7 3 ) prompted us to make kn0Lz.n our results in this area. T h e boron-containing compounds which were investigated include the borazines, a Present address, Department of Chemistry, Marquette University, Milwaukee, Wis.
CH 2-CH-CH
2-0-CH,-CH
2-CH
\ /
I ?-Si-O-SiI CHZ
0
FHA I
CH3
CH2-CH2-CH2-O-CH?-CH-CH,
\'
Sylkem 90
T h e properties of the various boron-containing compounds examined are listed in Table I. The borazines are cyclic compounds containing three atoms each of boron and nitrogen distributed about a planar ring. They \yere synthesized by
Table 1.
Boron Compounds Tested as Curing Agents for Epoxy Resins Formula Properties ( 79j B3C13N3H3 1f.p. 84.5-5.5 ' C. B3C13N3(CH3), M.p. 162-4 C.
Comfiound
B-Trichloroborazine B-Trichloro-.V-trimethyl-
borazine Benzeneboronic acid Trimet hoxyboroxine
CGHjB(0H)r (CH30B0)3
pij =
Tri-n-butoxyboroxine
(n-CaHgOBOj 3
p$j =
Methvlpolyborate (MPB-11)
Present address, Department of Chemistry. IYisconsin State Colleye, Oshkosh, \Vis. VOL.
1
NO. 3
M.P. 208-13" C. 1.25, n2d = 1.3986, v2j = 13 0 cs.
0.99, n2$ = 1 4200, v?j = 11 8 cs. p n j = 1.31: ny = 1.4135, Y?@ = 22,350 cs.
SEPTEMBER
1962
185
Table II.
Mechanical and Electrical Properties of Epon Resin 828 Boron Castings
Cure
P.H.R.4
Hr.
6
2 2d
Flexural Strength,&
P.s.I.
C. 150 200
18
13,800-15,000
200
3 3d
15.500
70 200
14.700-15,000
Frequency,c
C.P.S. 102 103 104 105 102 103 104 106 102 1.. 03
104 105
15,600
102
103 104 105 CeH5B(OH ) z
9
SIPB-I1
6 8d
200 150
5
150
115
17,500
8,300-9,700
102
103 104 105 102 103 104
m-Phenylenediamine
a
14
5
Parts ber hundred parts of resin.
b
150
17,000
A S T M procedure D 790-49T
the reaction of boron trichloride and the corresponding amine ( 7 , 5-9, 78). Similarly, the boroxines (metaborates) are cyclic compounds containing boron and oxygen synthesized by the dehydration of monosubstituted boric acids (74: 76) or the reaction of boric oxide and a trisubstituted borane (3, 75). Equations representing typical preparations of these compounds are:
c1
I
B 3BCIj
+ SRNHZCI
-+
9HCl
+ R--N / \y-R C1-B
I
1
(1)
B-CI
\ / N
I
R B-Trichloro-.V-trisubstitutedborazine
RMgX
+ (Me0)SB
c
ASTAVlprocedure D 64845T.
Curing Agent"
R-B
B-R
(2)
\o/ Trisubstituted boroxine Some typical data on the properties of unfilled cast polymers of Epon Resin 828 and the various boron-containing compounds are given in Tables I1 and 111. T h e trichloroborazines and the benzeneboronic acid were mixed with the resin at 100' C. prior to the initial cure. Lack of solubility of the chloro186
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
d
Dissipation Factorc
4.2 4.5 4.4 4.7 3.9 3.9 3.9 3.9 3.7 4 8 4:i 4.7 4.4 4.4 4.3 4.3 3.6 3.6 3.6 3.6 4.4 4.5 4.4 4.3 4.6 4.5 4.3 4.2
0.0108 0.0026 0,0042 0.0053 0.0049 0.0059
0,0051 0.0083 0.0108 0,0078 0.0086 0.0096 0,0028
0.0025 0,0028 0.0059 0.0011 0.0012 0,0026 0,0061
0,0026 0,0015 0,0032 0,0054 0.0072 0.015 0,029 0.037
Postcure.
borazines in Epon Resin 828 prevented the incorporation of more of these curing agents into the resin. Benzeneboronic acid produced a considerable amount of foam during the cure at 150' C. This may be caused by the release of volatiles or by the conversion of part of the acid to the anhydride with the release of water. All boroxines rested were miscible \vith Epon Resin 828 at room temperature; ho\vever, the trimethoxyboroxine possessed a considerably shorter pot life than the rributoxyboroxine. Some deflection temperatures of the system (C4H,OBO) 3Epon Resin 828 are given in Table IV. These data indicate that the best deflection temperatures range from 80" to 120" C. T h e methylpolyborate system (MPB-11) is interesting in that it represents the incorporation of a n inorganic polymer into a n organic resin. Pot life at room temperature is of the order of 30 minutes; however. because of its high viscosity.
Table 111.
-+
105 10' 103 104 105
Dielectric Constante
Chemical Properties of Boron-Cured Epon Resin 828 Castings
Wezght Lossb Days %
BJCI~N~H~ 6 B ~ C I S N ~ ( C H7~ ) ~ (C,HgOB0)3 7 CsHsB(OH)2 7 MPB-I1 7 m-Phenylenediamine 9
3.05e 6.40 3.44 1.55 1.74 3.00
Increase i n Weicht. - , % " 30% 70% Humidztyc H2SOa AVaOHd
0.53 0.51 0.93 0.54 0.78
0.13
0.66 0.81 0.30 0.79 0.llf
0.77 0.64 0.76 0.50
1.22 0.010
All tests performed on cured samples 5 X 5 X 3.5 cm. following A S T M Procedure D 547-52T. Weight loss in constant draft oven a t 200" C. c Exposure of 24 hours at 700' F. and 10070 humidity. d Seven daw' immersion at ambient temberature. 6 Sambles darkened and deformed Juring last 24-hour exposure' of 6-day test. f 70% HzSOa. 0 50% .VaOH. a
preheating of the resin to obtain a uniform solution was necessary. T h e exotherm of a solution containing 12 grams of Epon Resin 828 and 1.2 grams of MPB-I1 peaks in 20 minutes at 47' C. Some deflection temperatures of this system are given in Table V. T h e data indicate that prolonged cures a t 200' C. yield a deflection temperature in the range of 143' to 158'C. T h e pot life of the boroxines was extended through the use of a boroxine-amine complex. A number of these complexes have been discussed by Burg (2) and Snyder (77). Their data indicate that a 1 to 1 boroxine-amine complex is readily formed a t room temperature. T h e complexes (CIH,OBO) 3 : (C4H9)2NH and (C4HBOBO)3: ( C 2 H J 2 N H reached an equilibrium viscosity of 305 and 220 c.P.s., respectively, after 4 days at 25' C. When 15 p.h.r. of each of the above complexes \vas incorporated into Epon Resin 828, the resulting solution \vas still fluid after 6 rnonths at room temperature. A subsequent 15- to 20-hour cure at 175' to 200' C. was necessary to obtain polymers yielding flexural strengths in the range of 10,000 p.s.i. Similar behavior was observed with the corresponding complexes of the polyborates. While the borazines and the substituted boric acids may find limited use because of their low solubility in the epoxy resins, the polyborates and the boroxines yield homogeneous solutions with the epoxy resins at room temperature and react to form cross-linked polymers Lvhose properties are comparable to those obtained with commercially available curing agents. Because of the uncertain degree of purity of the commercially available boroxines and polyborates, no great significance can be attached to the somewhat high values of the dielectric constants obtained on the cured products. Of greatest interest perhaps is the emergence from this work of truly homogeneous, stable, latent curing agents in the form of the boroxine-amine complexes. Tables I1 to IV contain for comparative purposes data on Epon Resin 828 and the curing agent, m-phenylenediamine ( 72). Literature Cited
(1) Brown, C. A, Laubengayer, A. W., J. A m . Chem. SOL.77, 3699 (1955). ( 2 ) Burg, A. B., Ibid., 62, 2228 (1940). (3) Goubeau. J., Keller. H., 2. anorg. u. allgem. Cizem. 267, 1 (1957). (4) Greenlee, S. 0. (to Devoe and Raynolds Co.), U. S. Patent 2,717,885 (Sept. 13, 1955). (51 Groszos. S. J.. Stafiei. S. F.. J.A m . Chem. SOL.80. 135' 11958). (6j Hohnstddt, L.'F., H&orth,'D. T., Ztzd.. 82, 89 (i96O). (7) Jones, R. G., Kinney, C. R., Ibid., 61, 1378 (1939). (8) Kinney, C. R., Kolbezen, M. T., Zbid.;64, 1584 (1942). (9) Kinney, C. R., Mahoney, G. L.. J . Ore. Chem. 8, 526 (1943). (10) Langer, S. H. (to IQestinghouse Llrctric Corp.), U. S. Patent 2,871,454 (Jan. 27, 1959). \
I
Table
IV.
Deflection Temperaturesa of Epon Resin 828b Castings Cured with ( 1 t - C ~ H ~ 0 8 0 ) ~ Cure Cycle
P.H.R. 10 10
Hr. 1.5 3 3 3 19 3 3 3 3 16 3 3 3 26 3 3 3 3
4?;;
c.
200 48 150 68 100 10 150 96 180 14 150 80 190 14 150 120 190 240 18 150 90 200 18 150 114 200 22 150 74 200 30 150 50 200 a A S T M procedure D 6 4 8 - 4 5 T (stress of 264 p.s.i. ). m-Phenylrnediaminegaue D T of 150'C. using 74p.h.r., cured at 150' C. f o r 5 hours. Table
V.
Deflection
Temperaturesu of Epon Resin 828
DT,
P.H.R. 8 10 15
System
MPB-II-
C.
Cure cycle Ab
Cure cycle Bc
114 ...
1.. 5.8
113 107 103
154 152 143
20 A S T M procedure D 6 4 8 - 4 5 T (stress of 261p.s.i.). Initial curr, 2 hours at 750" C.; postcure, 4 hours at 200" C. Inilial cure, 2 hours at 750' C.; postcure, 20 hours at 200' C.
(11) ;anger, S. H., Elbing, I. N., IND.Erc. C m v . 4 9 , 1113 (1937). (12) Lee, H., Neville, K., "Epoxy Resins." McGraw-Hill, N e i v York, 1957. (13) Lee, H.. Neville, K.. SPE Journal 16, 315 (19pO). (14) McCusker, P. A , . Ashby, E. C., Makokvski, H. S., J . .4m. Chem. SGC. 79, 5179 (1957). (15) McCusker. P. X.: Ashby, E. C., Rutkoivski, .\. J., Ihid., 79, 5194 (1957). (16) Snyder, H. R., Kinecky. M. S., Lennarz, Ll'. J., Ibzd., 80, 3611 (1958). (17) Snkder, H. R.. Kuck, 3. A , . Johnson, J. R., Ibzd.. 80, 3611 (1958). (18) Turner, H. S., Fl'arne. R. J., Chem. &? Ind. (London) 1958, 526. (19) U. S. Borax and Chemical Corp., technical bulletins RECEIVED for review November 3. 1961 ACCEPTED May 14 1962
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